The present invention relates generally to a non-electrical detonating cap assembly and more particularly is concerned with a new and improved cap assembly and a bushing incorporated therein to provide a static charge shunt protecting the assembly from premature initiation.
In recent years detonating caps have been used in combination with fuses of the type consisting of a flexible plastic tube having an inner channel wall coated with a fine granular powder of reactive substances adapted to support a gaseous percussion wave throughout the length of the tube. Fuses of this type have been described in U.S. Pat. No. 3,590,739 and consist of flexible plastic tubing having a thin layer of explosive mix occupying only a small fraction of the fuse core, preferably as a part of the coating on the interior wall of the tubing. It has been reported that the explosive powder mix in such fuses has a tendency to detach from the inner wall of the fuse and accumulate on the top pyrotechnic charge within a detonating cap secured to the end of the fuse. This accumulation is reported to have prevented appropriate ignition of the cap. In order to correct this condition, it has been proposed that a spacer element be inserted between the free end of the tube and the top or delay charge within the detonating cap. This spacer element provides a shield that directs the loose powder away from the center of the top charge. Such a construction is disclosed in greater detail in U.S. Pat. No. 3,817,181 and is alleged to have the effect of assuring ignition of the cap.
The present invention relates to a different problem associated with fuses of the type described, namely the accidental initiation of the top charge due to the accumulation and flow of static charge along the length of the fuse. As mentioned, the fuse employs a plastic tubular member such as a polyethylene tube that is generally of electrically insulative character. It has been found that static charge accumulates on such a plastic fuse and will, in turn, induce a charge on the interior wall of the fuse tubing until that charge exceeds the breakdown potential of the powder film coating on the interior thereof. At this point the film becomes conductive and the charge flows along the length of the tube and into the cap causing premature ignition of the pyrotechnic delay element or booster charge and detonation of the cap. As will be appreciated, such a situation presents a substantial safety hazard.
Accordingly, it is an object of the present invention to provide a cap assembly that overcomes the aforementioned hazard and permits safe handling of fuse and cap assemblies of the type described heretofore. Included in this object is the provision for an assembly that permits full unhindered operation of the fuse and cap assembly under normal operating conditions yet isolates the pyrotechnic and explosive components from any static charge accumulation that might be present on the plastic fuse.
Another object of the present invention to provide a static charge shunt for a detonating cap assembly that includes a circuit or path of low resistance through which static charges may flow prior to reaching the pyrotechnic and explosive components of the cap. Included in this object is the provision for an interconnecting bushing between the cap and the fuse that provides a charge barrier. At the same time it prevents either contamination of the delay element by the material within the fuse or contamination of the end of the fuse by powdered material loosened from the pyrotechnic or delay charge.
Another object of the present invention is to provide an assembly of the type described that includes not only a charge barrier but also a low resistant shunt path or circuit between the end of the fuse positioned within the cap and the outer cap shell of the assembly.
Other object will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a new and improved detonating cap assembly that includes an elongated shell open at one end for receiving a fuse, a charge in the shell spaced from the open end and an elongated bushing in the open end interconnecting the fuse to the shell of the detonating cap. The bushing has a confined end positioned adjacent the charge, a longitudinal bore extending from an exposed end to a location adjacent the confined end and a rupturable membrane at the confined end terminating the bore. The bushing further includes a low resistance shunt path adjacent the membrane and intermediate the membrane and the exposed end of the bushing. The shunt extends laterally outwardly from the bore toward the shell for diverting static charge toward the shell.
A better understanding of the invention will be obtained from the following detailed description and the accompanying drawing of an illustrative application of the invention.